Lightly grease the sides and bottom of a 9" round springform pan with cooking spray. Set aside.

In the bowl of a stand mixer, beat the heavy whipping cream on HIGH power with the whisk attachment until soft peaks form, about 4-5 minutes. Add in the confectioners' sugar and dry cake mix powder and beat until stiff peaks form, about 2 minutes longer. Stir in the rainbow sprinkles. Pour the whipped cream mixture into a medium bowl and set aside.

In the same bowl of a stand mixer, cream together the cream cheese, granulated sugar and vanilla until smooth and blended, about 1 minute. Fold the whipped cream mixture back into the cream cheese mixture until no streaks remain and mixture is fluffy and fully incorporated. Spread about 1/4 cup of the mixture onto the bottom of the springform pan. This just helps adhere the first layer of Oreos.

Dip an Oreo one at a time into the milk and layer around the bottom of the pan, breaking Oreos to fit if necessary. Spread 1/3 of the mousse mixture over the top with an offset spatula. Repeat with the Oreo layer, then another 1/3 of the mousse layer. Repeat layers once more, finishing with the remaining birthday cake mousse.

Cover and refrigerate for at least 4-6 hours or overnight. Just before serving, garnish with additional whipped cream and rainbow sprinkles.